It is known to provide for such an adjustment at least one motorized drive which is associated with the head support. In the case of a motorized drive, the head support is positioned in the adjusted disposition, against an unintended movement, due to the self-locking of the motorized drive. For the purpose of reducing the mechanical outlay there have, however, also been developed manually operable adjustment devices for the head support. With this it is necessary to fix the head support in the respective adjusted position, against an unintended displacement, by means of a fixing device. With regard to the state of the art attention is directed to DE 82 07 541 U1, DE 296 20 801 U1, EP 0 701 806 A1 and EP 0 673 633 A2.
A treatment chair of the kind indicated in the preamble of claim 1 is described in DE 29 32 345 A1. With this known treatment chair, a head support is connected in a middle position by means of a base joint with the free end of a telescopic support part which at its other end is pivotably connected by means of a joint with a backrest of the treatment chair. For pivoting the telescopic support part in its base joint there is provided a piston-cylinder unit which is jointed on to the backrest at a radial spacing from the base joint and engages in a jointed manner with its other end at the forward end of the telescopic support part. There is associated with the telescopic support part a latching device which makes it possible selectively to latch the telescopic support parts to one another or to release them. The base joint can be selectively locked or released with a second latching device. With this known treatment chair it is possible, depending upon the actuation of the first or second latching device, to extend or retract the head support by means of an appropriate actuation of the piston-cylinder unit or to pivot the head support around the joint axis of the base joint, the head support itself being freely pivotably in the joint connecting it with the telescopic support part. This known configuration is complex both with regard to construction and also operation. Further, the support function is unsatisfactory.